1. Field of the Invention
This invention relates to the photolysis of water using a p-type semiconductor cathode comprising a rhodate and an anode comprising an n-type semiconductor or a metal conductor. When n-type TiO.sub.2 is used as the anode solar radiation not only decomposes the water into oxygen and hydrogen but also produces an electrical current.
2. Prior Art
The photoelectrolysis of water was first observed by Hondo and Fujishima, Nature 238, 37 (July 7, 1972). When an n-type semiconductor, used as an anode in an electrolytic cell containing an aqueous electrolyte, is irradiated with greater than band-gap light, the applied potential required to electrolyze water is reduced. Such photoassisted electrolysis has been the subject of much recent work. Hondo and Fujishima suggested that a more effective reduction of the applied potential might arise in a cell containing both an n-type anode and a p-type cathode.
U.S. Pat. No. 4,011,149 to Nozik discloses thin-film electrodes one of which can be a p-type semiconductor and the other an n-type semiconductor.
U.S. Pat. No. 3,925,212 to D. I. Tchernev discloses a system which utilizes an n-type semiconductor such as TiO.sub.2 and a p-type semiconductor such as GaP. H. Yoneyama et al., Electrochim. Acta. 20, 341 (1975) disclose a similar system and observe that deterioration of the cell performance occurs, mainly due to the instability of the p-GaP electrode. A. J. Nozik, Applied Physics Letters 29 No. 3, 150 (Aug. 1, 1976) likewise discusses a system utilizing an n-type semiconductor such as TiO.sub.2 and a p-type semiconductor such as GaP and gives an energy analysis of such a system. None of these systems are very stable, and in fact GaP has been the subject of much research on its dissolution in alkaline and acid solution [R. Memming and G. Schwandt, Electrochimica Acta 13, 1299 (1968)].
U.S. Pat. No. 3,632,498 to Beer discloses an electrode part of whose surface may comprise rhodium oxide. The patent makes no mention of the properties of such rhodium oxide and has no teaching of a p-type semiconducting trivalent oxide. Further, no disclosure is made of any solar cell in using a p-type trivalent rhodium oxide as cathode.
U.S. Pat. No. 3,644,147 to Young discloses a number of mixed valency transition metal perovskites as cathodes for fuel cells or electrolytic cells wherein electrical energy is supplied from an external source (Col. 1, lines 44-45). The nature of the perovskites as to whether they are n-type or p-type, etc., is not mentioned. And again no mention is made of a solar cell using a p-type trivalent rhodium oxide as cathode.